Spark plugs that have electrodes that are provided with tips or pads formed of noble metal, such as platinum, are well known to those skilled in the art. A known method for forming pads of platinum material on a spark plug electrode utilizes a contact welding method in which a platinum wire is engaged with the surface of the spark plug electrode and a weld current is passed through the platinum wire causing the portion of the wire that engages the electrode to be welded to the electrode. The wire is then cut off so that a pad or tip of material remains that is welded to the electrode.
In a known system for implementing the welding of platinum wire to spark plug electrodes, a turn table is provided with several clamp assemblies. Within each clamp is provided a spark plug electrode and the spark plug electrodes are one-by-one rotated on the turntable into a welding position, the clamps are raised, an electrode is brought into contact with the raised clamp, and the welding operation is commenced.
Referring to FIG. 1, an illustration of a prior art clamp contact with the spark plug electrode and part of the weld circuit path is shown. Spark plug electrode 45 is a cylindrical electrode having an outside (reference 46) formed of a nickel based alloy, such as Inconel 600, and an internal copper core 48. According to the illustrated prior art, a large area 47 of the side of the electrode is contacted by the conductive portion 50 of the clamp so that when the wire 40 contacts the top of the electrode and current is provided therethrough along path 41, the portion of the wire 44 contacting the upper tip of the electrode melts and welds to the upper tip of electrode 45.
Lines 43 illustrate the current flowing through the electrode and show that much of the current may flow through the copper core 48. Because process variations allow for variations in the positioning of copper core 48, the weld circuit impedance varies from electrode-to-electrode, as the positioning of copper core 48 varies.
FIG. 2 shows a top view of a prior art clamping device for the electrode.
FIG. 3 illustrates a prior art weld circuit where resistance 54 represents the upper electrical lead from the weld current supply transformer. Resistance 56 represents the resistance of the platinum wire collet and its clamping around the platinum wire. Resistance 58 represents the platinum wire resistance. Resistance 60 represents the interface between the platinum being welded and the center electrode. Resistance 62 represents the resistance within the center electrode. Resistance 64 represents the resistance between the clamp and the center electrode. Resistance 66 represents the resistance within each clamp. Resistance 68 represents the resistance of the connector between the clamp and the lower electrical leads and resistance 70 represents the lower electrical leads coupled between the clamp and the weld current supply.
The resistances 56, 60, 62, 64 and 68, which are shown with surrounding circles, represent resistances that vary from cycle-to-cycle due to the nature of the process. The resistance 56, including the point of contact between the collet and platinum wire, naturally varies from cycle to cycle. This is also true of the resistance 60, representing the interface between the melting wire and the center electrode. Because the position of the copper core of the center electrode varies from electrode, to electrode, the resistance 62 of the center electrode varies from cycle to cycle. The point of clamp contact to the center electrode, represented by resistance 64, naturally varies from cycle to cycle. Because a contact must be made with each clamp in the weld station, the resistance 68 at the point of contact with the clamp and the electrical lead varies from cycle to cycle.
The resistances 64, 66 and 68, each indicated with a surrounding square, vary from clamp to clamp. In total the, Figure illustrates eight varying parameters in the above described welding system.